This invention relates to the transmission and reception of television programs and, more particularly, to the multiplexed transmission of two television programs via the same communication link.
Copending U.S. Ser. No. 808,908 filed Dec. 13, 1985, is assigned to the present assignee and is hereby incorporated by reference. Briefly, the foregoing application discloses a multiplexing system in which two independent television programs, preferably in NTSC format, can be reformatted to fit a signal NTSC TV channel having a conventional base bandwidth of 4.2 MHz.
The preferred embodiment described by the above-referenced application combines the two successive video fields of an NTSC frame to form a pair of signals. One of the signals is the sum of each pair of adjacent frame lines, while the other signal constitutes the difference between the pairs of adjacent frame lines. The sum signal and difference signal of each line pair are each scaled; if a line of the first field is labelled "A" and the adjacent line of the second field is labelled "B", then the scaled sum and difference signals for the line pair can respectively be represented as 1/2(A+B) and 1/2(A-B).
The scaled difference signal in the aforementioned preferred of 4 by low pass embodiment is band-reduced by a factor filtering, thereby reducing the difference signal's bandwidth to approximately 1.05 MHz. The low-pass filtered signal is then time compressed by a factor of 4 before transmission.
The scaled sum signal of each line pair is serially combined with the time-compressed difference signal of the line pair, a chroma subcarrier reference burst, and a gray scale reference signal. The resulting line-pair signal, formed for each line pair, is transmitted serially with the line-pair signals of the other line pairs in the frame.
The preferred encoder described by the referenced application produces the line-pair signals of the encoded video frame during the time interval allotted for transmission of a video field. Because the frame has been transmitted during half the time allotted for its transmission under the NTSC broadcast format, an encoded frame of a second television program may be transmitted during the remaining field interval.
The transmitted signal, preferably comprising encoded frames of both programs in alternating succession, is received by a decoder, which separates the signal into its component parts. The original picture format of each frame is preferably reconstructed by time-expanding the compressed difference signal, smoothing it in an interpolation or reconstruction filter, and recombining it with the sum signal to reform both of the originally scanned fields. One field is reconstructed by summing the sum and difference signals while the other field is reconstructed by subtracting the difference signal from the sum signal. After this reconstruction, the signals are substantially identical to the original premultiplexed signals.
The instant invention is directed to an improved multiplexing system which avoids the use of complex digital filters. Additionally, the disclosed improvement includes a memory configuration which is shared by both transmitted programs, thereby reducing memory requirements.
It is well known that complex digital filters can be used both as anti-alias filters before sub-sampling and time compression in the encoder portion of a multiplexing system, and as interpolation or reconstruction filters in a demultiplexer. Filters having a flat response and steep cutoff at the highest transmittable frequency provide the best performance. Such filters are typically known as "wall filters" and are complex in design and expensive to produce. Accordingly, the performance of multiplexing systems has generally been compromised by selecting simpler filter designs having gradual frequency roll-offs and requiring fewer components in their implementation.